As a radiation detector which measures radioactive substances, various types of devices are known. The Geiger counter is the most primitive radiation detector. Also, the radiation detector applied with the principle of a pinhole camera is also known. In case of the radiation detector of such a pinhole camera type, a measurement range is wide to an extent (viewing angle=about 60 degrees, measurable distance=ten and several meters) and a distribution of the radioactive substances in the measurement range can be measured through once measurement.
The applicant of the present application is developing “the Compton camera” which is more highly efficient than the pinhole camera (for example, reference to Patent Literature 1, Patent Literature 2). The Compton camera utilizes the principle of Compton scattering in which gamma radiation has the nature of a particle. In case of the Compton camera, the viewing angle is wide to be 180° (strictly, 2π steradian in a solid angle) and also the measurable distance is long to be about 30 m. In other words, the measurement range of the Compton camera is much wider than the cameras of other types. Also, the Compton camera measures the energy of the radiation and can identify a kind of the radioactive substance (nuclide) based on the energy measurement data.